U.S. patent application number 12/530558 was filed with the patent office on 2010-04-15 for driver circuit for loads such as led, oled or laser diodes.
This patent application is currently assigned to KONINKLIJKE PHILIPS ELECTRONICS N.V.. Invention is credited to Carsten Deppe, Matthias Wendt.
Application Number | 20100091807 12/530558 |
Document ID | / |
Family ID | 39615850 |
Filed Date | 2010-04-15 |
United States Patent
Application |
20100091807 |
Kind Code |
A1 |
Deppe; Carsten ; et
al. |
April 15, 2010 |
DRIVER CIRCUIT FOR LOADS SUCH AS LED, OLED OR LASER DIODES
Abstract
A driver circuit 10 is described for driving loads such as LED,
OLED or LASER diode devices L. A switching converter 12 has a
switching element M1 and reactive elements L1, C1 to provide an
output switching voltage V1 by sequential switching operations of
the switching element M1. The load L is connected to the output
switching voltage. A linear current driver circuit 14 is connected
in series to the load L and comprises an amplification element Q1
and a feedback circuit R1, 22 with a current control input V.sub.L,
set, I.sub.B. In order to enable the circuit to be easily used, a
control unit 16, 116, 216 is provided with a sensing input V.sub.L,
1, V.sub.L, 2 for a current or voltage at the linear current driver
14. A microcontroller 30, 130, 230 executes a control program for
processing the sensing input and providing both a current control
output V.sub.L, set, I.sub.B and a switching control output
V.sub.L, set in accordance with a set current value I.sub.set.
Inventors: |
Deppe; Carsten; (Aachen,
DE) ; Wendt; Matthias; (Wuerselen, DE) |
Correspondence
Address: |
PHILIPS INTELLECTUAL PROPERTY & STANDARDS
P.O. BOX 3001
BRIARCLIFF MANOR
NY
10510
US
|
Assignee: |
KONINKLIJKE PHILIPS ELECTRONICS
N.V.
EINDHOVEN
NL
|
Family ID: |
39615850 |
Appl. No.: |
12/530558 |
Filed: |
March 12, 2008 |
PCT Filed: |
March 12, 2008 |
PCT NO: |
PCT/IB2008/050887 |
371 Date: |
September 9, 2009 |
Current U.S.
Class: |
372/38.04 ;
315/224 |
Current CPC
Class: |
H05B 45/3725 20200101;
Y02B 20/30 20130101; H05B 45/37 20200101; H05B 45/46 20200101; H05B
45/20 20200101; H05B 45/375 20200101 |
Class at
Publication: |
372/38.04 ;
315/224 |
International
Class: |
H01S 3/00 20060101
H01S003/00; H05B 37/02 20060101 H05B037/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 15, 2007 |
EP |
07104191.7 |
Claims
1. Driver circuit for an LED, OLED or laser diode device (L), the
circuit comprising a switching converter comprising at least one
switching element, at least one reactive element (L.sub.1, C.sub.1)
and an output (V.sub.1), said switching converter being configured
to generate a switching output voltage (V.sub.1) by sequential
switching operations of said switching element (M.sub.1), a
terminal for connecting said at least one LED, OLED or laser diode
device (L) to said output (V.sub.1), a linear current driver
connected in series to said terminal, said linear current driver
comprising an amplification element (Q.sub.1), a current sensing
means (R.sub.1) and a current control input to control a drive
current (I.sub.L) for said terminal, and a control unit,
comprising: a sensing input for sensing a current and/or voltage
(V.sub.L, 1, V.sub.L, 2) at said linear current driver, a switching
control output (V.sub.1, set) for controlling said switching
converter, a current control output (V.sub.L, set, I.sub.B) for
providing a current control signal to said linear current driver,
and a programmable control means executing a control program for
processing said sensing input and providing a switching control
output (V.sub.1, set) and a current control output (V.sub.L, set,
I.sub.B) in accordance with a set current value (I.sub.set);
wherein said set current value (Iset) comprises one ore more
variable portions where the value changes over time, and one or
more steady portions where the value remains substantially
constant, and wherein said control program is configured to provide
a switching control output (V1, set) to increase said switching
output voltage (V1) before the variable portion.
2. Circuit according to claim 1, wherein where at least in a part
of said steady portions, said control program is disposed to
provide a switching control output (V.sub.1, set) such that a
voltage (V.sub.L, 1) over said linear current driver is controlled
to be at a threshold value (V.sub.threshold).
3. Circuit according to claim 1, wherein said control unit provides
a desired output switching voltage (V.sub.1, set) and said
switching converter comprises a feedback controller to control the
output voltage (V.sub.1) to the desired output switching voltage
(V.sub.1, set).
4. (canceled)
5. Circuit according to claim 1, wherein in said variable portion
(36, 38), the set current value (I.sub.set) rises, and remains at
least essentially constant in a following steady portion, and
wherein set control program is disposed to provide switching
control output (V.sub.1, set) to raise said switching output
voltage (V.sub.1) to a higher level before said variable portion,
and to a lower level within at least of said following steady
portion.
6. Circuit according to claim 1, wherein said control unit
comprises storage means for storing a plurality of desired
switching output voltage values (V.sub.1, set) and desired drive
current values (I.sub.set), said program being configured to (i)
retrieve values from said storage means for determining said
desired switching output voltage (V.sub.1, set) for a set current
value (I.sub.set) and (ii) to store determined values (V.sub.1,
set, I.sub.set).
7. (canceled)
8. Circuit according to claim 1, wherein a plurality of branches
are connected in parallel to said output (V.sub.1), each branch
comprising at least one terminal for connecting an LED, OLED or
laser diode device (L), in each branch there is provided a linear
current driver circuit connected in series to said terminal, and
said control unit comprises at least one sensing input for sensing
a current and/or a voltage within each of said branches.
9. Circuit according to claim 8, wherein said control program is
configured to process said inputs from said branches and set
current values for each of said branches, and said switching
control output is determined according to the minimum of sensed
voltage values at the linear current drivers of each branch.
10. Method for operating at least one LED, OLED or laser diode
device (L), comprising generating a switching output voltage
(V.sub.1) in a switching converter comprising at least one
switching element (M.sub.1) and at least one reactive element
(C.sub.1, L.sub.1), by sequential switching operations of said
switching element (M.sub.1), providing said output switching
voltage (V.sub.1) to at least one LED, OLED or laser diode device
(L), controlling a drive current for said device (L) using a linear
current driver circuit connected in series to said device (L), said
linear current driver comprising an amplification element
(Q.sub.1), a current sensing means (R.sub.1) and a current control
input (V.sub.L, set, I.sub.B), providing a desired switching output
voltage (V.sub.1) and a desired drive current (I.sub.L) in
accordance with a set current value (I.sub.set) by executing a
control program in a programmable control means processing at least
one sensing input from a current and/or a voltage at said linear
current driver and controlling said switching converter to deliver
said switching output voltage (V.sub.1), wherein said set current
value (Iset) comprises one ore more variable portions where the
value changes over time, and one or more steady portions where the
value remains substantially constant, and wherein said control
program is configured to provide a switching control output (V1,
set) to increase said switching output voltage (V1) before the
variable portion.
11. Driver circuit for an LED, OLED or LASER diode device (L), the
circuit comprising a switching converter comprising at least one
switching element, at least one reactive element (L1, C1) and an
output (V1), said switching converter (12) being configured to
generate a switching output voltage (V1) by sequential switching
operations of said switching element (M1), a terminal for
connecting said at least one LED, OLED or LASER diode device (L) to
said output (V1), a linear current driver connected in series to
said terminal, said linear current driver (14) comprising an
amplification element (Q1), a current sensing means (R1) and a
current control input to control a drive current (IL) for said
terminal, a control unit, comprising: a sensing input for sensing a
current and/or voltage (VL, 1, VL, 2) at said linear current
driver, a switching control output (V1, set) for controlling said
switching converter, a current control output (VL, set, IB) for
providing a current control signal to said linear current driver,
and a programmable control means executing a control program for
processing said sensing input and providing a switching control
output (V1, set) and a current control output (VL, set, IB) in
accordance with a set current value (Iset), wherein said control
unit comprises storage means (56) for storing a plurality of
desired switching output voltage values (V1, set) and desired drive
current values (Iset), and wherein said program is configured to
store values (V1, set, Iset) determined during control into said
storage means, and to retrieve said stored values from said storage
means for determining said desired switching output voltage (V1,
set) for a set current value (Iset).
Description
FIELD OF THE INVENTION
[0001] The present invention relates to driver circuit for a load
such as LED, OLED or LASER diodes and a method for operating one or
more of such loads.
BACKGROUND OF THE INVENTION
[0002] Light emitting diodes (LEDs) are used today in a plurality
of lighting and display applications, where they are preferred over
conventional lamps due to significant advantages such as high
energy efficiency and long operating life. A special type of LEDs
are organic light-emitting diodes (OLED). Another type of
electrical load that is targeted by the present invention are LASER
devices.
[0003] Regarding the demands posed on driver circuits for loads
such as LED, OLED and LASER diodes, these electrical loads require
very accurate on-current. In some applications, lighting units are
driven in a pulsed manner. It is thus important for the driver
circuit to be able to provide current pulses with accurate
on-current, minimal pulse distortion, low raise and fall times and
low overswing.
[0004] In many lighting and display applications, the loads are
operated with pulse patterns. For example, this may be used to
control the brightness by techniques such as PWM (pulse width
modulation) or PDM (pulse density modulation). If the switching
frequency is high enough, the human eye will integrate the
brightness and perceive a mean brightness.
[0005] Also, pulses may be used in display applications with
sequential colour rendering. In order to use monochromatic light
modulation devices, such as, e.g. DMD (digital micromirror device)
or DLP (digital light processing) for displaying colour images, the
devices are sequentially used for different colour lights. The
light, in this case, may be provided by LED, OLED or LASER diodes
sequentially driven with short pulses.
[0006] Known electrical circuits for driving such loads include on
one hand linear mode driving circuits. Such linear mode driving
circuits are known to the skilled person and may be implemented in
many different ways. A linear current driver comprises an
amplification element (such as, for example, an operational
amplifier, a transistor, MOSFET or other comparable component) and
a current sensing means for sensing a current through the driver
and controlling the amplification element to achieve an analogue
control with feedback.
[0007] Linear current drivers can be designed to have the advantage
of an excellent dynamic behavior, but are known to introduce high
losses.
[0008] Another known type of driving circuit is a switching
converter. Such a converter comprises at least one switching
element and a reactive element (such as an inductance or
capacitance, or both). An output voltage is generated by sequential
switching operations of the switching element. By modification of
the duty cycle, the output may be controlled. Switching converters
are known for high efficiency, but have limited dynamic
behavior.
[0009] In US 2006/0108933, LEDs are driven by a combination of a
switching converter and linear current drivers. The DC to DC
converter outputs a direct current voltage for feeding two LED
series connected in parallel. Each series comprises a constant
current circuit connected in series. Each constant current circuit
receives a control signal and controls the LED current accordingly.
An analogue feedback circuit compares feedback voltages for each
LED series and uses the lower of the two as a feedback voltage to
the DC to DC converter. The converter compares the feedback voltage
to an internal reference voltage and adjusts its output voltage
accordingly.
[0010] It is an object of the invention to provide a driver circuit
and operating method well suited for the mentioned loads, which
provides both reduced losses and exact control, especially for
pulsed applications, and may easily be used.
[0011] This object is achieved by a driver circuit according to
claim 1 and an operating method according to claim 10. Dependent
claims refer to preferred embodiments of the invention.
SUMMARY OF THE INVENTION
[0012] The inventive driver circuit on one hand comprises a
switching converter which generates a switching output voltage by
sequential switching operations of a switching element. A terminal
is provided for connecting a load, such as one or more LED, OLED or
LASER diode devices to the output voltage. On the other hand, the
driver circuit comprises a linear current driver connected in
series to (a load connected at) the terminal. Thus, the load is
supplied in series by the switching converter and the linear
current driver.
[0013] These elements both accept a control input. The switching
converter has a switching control input. By different signals
provided at the switching control input, the provided output
voltage is modified. The switching control input may either be
direct switching information, i.e. the specific on/off-states of
the switching element(s), or may be an analogue signal such as a
reference voltage or a control offset. The linear current driver
accepts a current control input, which may preferably be provided
as an analogue current or voltage signal determining the drive
current controlled by the driver.
[0014] According to the invention, a control unit is provided
comprising at least one sensing input and at least two control
outputs, namely a switching control output for controlling the
switching converter and a current control output for controlling
the linear current driver. The control unit comprises a
programmable control means executing a control program. The program
and the invention method work to drive the components of the
circuit to provide a current through the load connected at the
terminal which corresponds to a set current value. To achieve this,
the sensing input is processed and a desired switching output
voltage and a desired drive current are determined.
[0015] The programmable control means may be any type of device
suited to execute the corresponding control program. Specific
examples include microprocessors, signal processors, or most
preferably, microcontrollers comprising a central processing unit
and a additional peripheral components, such as inputs, outputs,
memory etc.
[0016] The sensing input processed by the program comprises at
least a sensing input for an electrical value (current and/or
voltage) at the linear current driver. This sensing input may
comprise sensing the current through the driver (e.g. as a related
voltage signal), but preferably also contains voltage information
about the amplification element within the linear current
driver.
[0017] A circuit according to the invention provides great
flexibility in obtaining a load current in accordance with the set
current value. From the outside, the driver circuit only needs the
provided set current value. The microcontroller takes care of
providing, for each state of operation, the corresponding control
for the two elements, namely the switching converter and the linear
current driver. The combination of these two allows to benefit both
from the excellent dynamic properties of the linear current driver,
and from the reduce losses at the switching converter. Still,
external control of the circuit is very easy.
[0018] As will become apparent, the inventive device and method are
well-suited for driving the load with current pulses.
[0019] In a preferred embodiment, the set current value is not
constant but changes over time. Specifically for pulses, there will
be change portions, where the value changes over time (in fact for
pulse applications, the change portions will be extremely short,
namely correspond to the rising and falling edges of the pulse
signal). Also, there will be steady portions, where the set current
value will remain constant, or at least substantially constant
(which is understood to mean a variation of no more than .+-.20%,
preferably .+-.5%). It should be noted that in many applications,
there is information available in advance about timing and/or
height of the pulses, e. g. in the case of a periodic pulse signal.
Information about periodicity of the set current value may be
provided from the outside in different forms, or may be acquired by
the program in a self-learning manner.
[0020] One of the preferred embodiments relates to control within
the steady portions. Here, during at least a part of the steady
portions (preferably a central part, i.e. not directly bordering on
a change portion), control is effected to minimize a voltage over
the linear current driver. This is due to the fact that the linear
current driver, if it effectively limits the load current by
increasing the voltage drop at the amplification element,
introduces high losses. While such losses may be tolerated for a
short time, e.g. within the change portions and in limited time
periods before and/or after, they should be minimized over a long
term. Thus, minimizing the voltage drop over the linear current
driver will significantly limit these losses. Preferably, however,
minimization is performed such that a lower limit value (minimum
threshold value) for the voltage is maintained, allowing the linear
current driver to still operate within its linear range. This
control behavior is achieved by the corresponding control program.
Preferably, as will be further explained with relation to preferred
embodiments, the minimum threshold value may be calculated such
that a minimum voltage remains for operation of the amplification
element within the linear current driver. The minimum threshold
value may thus be calculated dependent on the load current.
Alternatively, it is possible to determine a fixed value for the
minimum threshold value, and to store this value in (or make it
accessible to) the control means.
[0021] According to a further preferred embodiment, where the set
current value has change portions and steady portions as above,
control is effected to provide a higher desired switching output
voltage at least immediately before a change portion. Thus, for
example, before and optionally also (preferably shortly) after the
change portion, the desired switching output voltage will be lower
than within the time period comprising the change portion and the
time immediately before. This raising of the switching output
voltage takes account of the fact that the switching output voltage
cannot be raised instantaneously. Since the reactive elements e.g.
an output capacitor of the switching converter need to be charged,
some time is required for the switching converter to raise the
voltage. Thus, in order to provide the desired voltage value
already in a change portion, the change is initiated by the control
program in advance. Of course, in this case there needs to be
information available to the microcontroller about impending change
portions. However, this will often be the case, e.g. if the set
current value is changed according to a known pulse pattern. For
example, the pulse pattern may be supplied externally to the
control means as a digital signal. Also, for many applications, the
pulse pattern may be at least roughly periodical (so that pulse
timing and at least approximate pulse height are known in advance).
The periodicity may either be indicated by a signal at an external
interface, or may be recognized by a special procedure within a
control means. Most preferably, the pattern is supplied along with
a trigger signal indicating each start of a periodically repeated
pulse pattern.
[0022] It should be noted, that the raising of the switching output
voltage effected in advance of an impending change portion alone
has no significant influence on the load. This is, because the load
is still individually controlled by the linear current driver. The
excess part of electrical power supplied in this case is absorbed
as losses in the linear current driver. However, the dynamic
properties of the linear current driver may be fully exploited in
this way.
[0023] While this is also true for changes where the set current
value decreases in the change portion, it is especially applied for
change portion during which the set current value rises. If, for
example in a pulse application, the set current value rises and
then remains constant in a following steady portion, the control
program may provide a higher desired switching output voltage
before and during the change portion so that e.g. an output
capacitor is charged and the linear current driver may operate in
the linear range also for the higher set current value. Then,
within the steady portion, (at least during a later part of the
steady portion) the desired output switching value is again
lowered, as described above, to limit losses in the linear current
driver.
[0024] According to a further preferred embodiment of the
invention, the control unit comprises storage means for storing a
plurality of desired switching output voltage values and desired
drive current values. These storage means may be provided
internally or externally to the programmable control means, e.g.
microcontroller, and may comprise any type of digital storage
means. Especially preferred for fast access is a RAM storage,
preferably within the microcontroller. This storage is used by the
program to determine, for a given set current value, an appropriate
desired switching output value.
[0025] The storage means may be pre-programmed with predetermined
values. According to a further preferred embodiment, however, it is
preferred to have the program store values determined during
control. It is further preferred to use the stored values during
control as starting values only, and determine appropriate values
by subsequent closed-loop control. The thus given values then may
be stored in the storage means to update previous values. In this
way, the storage is always kept updated, such that any changes in
the driving circuit and/or the load may be compensated.
[0026] According to a further preferred embodiment, a plurality of
branches are provided. The branches are connected in parallel to
the output, such that they may all be driven by only one switching
converter and have a common switching output voltage. Each branch
has at least one terminal for connecting a load, such as an LED,
OLED or LASER diode device.
[0027] While for the plurality of branches there is only one
switching converter, each branch has an individual linear current
driver circuit connected in series to the terminal. Also, there is
an individual sensing input in each of the branches. Accordingly,
the control unit will have (direct or multiplexed) input terminals
for the sensing input from each of the branches. In this way,
multiple loads can be very effectively driven by a circuit with low
part count, where only one switching converter and only one control
unit with microcontroller is used for a plurality of loads. There
may be any number, such as 2, 3 or more loads connected. The loads
may be driven simultaneously, but for some applications--such as
sequential color rendering in projection applications--it is
preferred to drive the loads sequentially.
[0028] Preferably, in the case of a plurality of branches to be
driven simultaneously, the control program determines the desired
switching output voltage according to the minimum measured voltage
values at the linear current drivers of the individual branches.
Then, individual loads are controlled by the linear current drivers
in each branch.
BRIEF DESCRIPTION OF THE DRAWINGS
[0029] The above and other objects, features and advantages of the
present invention will become apparent from the following
description of preferred embodiments, in which:
[0030] FIG. 1 shows a circuit diagram of a driver circuit according
to a first embodiment of the invention;
[0031] FIG. 2 shows a circuit diagram of a driver circuit according
to a second embodiment of the invention;
[0032] FIG. 3 shows a timing diagram showing in schematical form
currents and voltages in FIG. 1 for a first operating mode;
[0033] FIG. 4 shows a timing diagram showing in schematical form
currents and voltages in FIG. 1 for a second operating mode;
[0034] FIG. 5 shows a schematic representation of a microcontroller
within the circuits of FIG. 1, FIG. 2;
[0035] FIG. 6 shows a circuit diagram of a driver circuit according
to a third embodiment of the invention;
[0036] FIG. 7 shows a timing diagram showing measured values of
currents and voltages in FIG. 6 in an example of a projection
system.
DESCRIPTION OF DETAILED EMBODIMENTS
[0037] FIG. 1 shows a circuit diagram of a driver circuit 10
connected to a load L.
[0038] In the example shown, the load L is an LED load, in this
case a series of four LEDs LED1, LED2, LED3, LED4. As will become
apparent, the driver circuit 10 may alternatively be used to drive
other devices, especially light-emitting devices such as OLED or
LASER diode devices. It is easily recognizable for the skilled
person how to connect other devices to the driver circuit 10.
[0039] The driver circuit 10 comprises a switching converter 12, a
linear current driver 14 and a control unit 16. The switching
converter 12 supplies a switching output voltage V.sub.1. The load
L and the linear current driver 14 are connected in series to the
output V.sub.1. The control unit 16 receives a set value I.sub.set
indicating the desired current for the operation of the load L. The
control unit 16 drives the components 12, 14 of the driver circuit
10 to achieve a load current. I.sub.2 which as closely as possible
follows I.sub.set.
[0040] The switching converter 12 is comprised of a switching
controller 20, which may be e.g. an integrated switching controller
of the type LT1765 available from Linear Tech-nologies. The
switching controller comprises a switching element M.sub.1 which
may be switched on and off according to a feedback signal received
at an input FB in. The switching converter further comprises a
diode D.sub.1, a series inductance L.sub.1 and an output
capacitance C.sub.1.
[0041] In the example shown in FIG. 1, the series inductance
L.sub.1 has an inductivity of 22 .mu.H, and the output capacitance
C.sub.1 is an electrolytic capacitor of 100 .mu.F. As the skilled
person will certainly appreciate, the shown components constitute
an exemplary embodiment only, and the switching converter 12 may be
implemented using quite different components. Specifically, the
shown topology of the switching converter 12, which is implemented
here as a buck converter, may be replaced by other known to
topologies of switching converters such as boost converters (if the
output voltage is above the input voltage), flyback (in- and output
have reverse polarities) or sepic.
[0042] The switching converter 12 further includes a feedback
controller 26. The feedback controller 26 generally may be of any
type suited to compare an actual voltage V.sub.1 to a set voltage
V.sub.L set and to provide a feedback signal FB accordingly. The
feedback controller 26 serves to control the output voltage V.sub.1
of the switching controller 12. As will become apparent later, in
the preferred modes of operation according to the invention, the
dynamic behavior of the output of switching converter 12 is already
assumed to be quite slow. Therefore, the controller behavior of
controller 26 need not be highly dynamic, and may be e. g. of an
integral type (I-controller).
[0043] The linear current driver 14, as shown in FIG. 1, has a
bipolar transistor Q.sub.1 acting as amplification element. A
feedback circuit is comprised of a series resistance R.sub.1 and an
operational amplifier 22. The operational amplifier 22 receives a
voltage input signal V.sub.L, set at its non-inverting input and a
feedback voltage V.sub.L, 2 at its inverting input. The feedback
voltage V.sub.L, 2 is dependent on the load current I.sub.2, which
also traverses the series resistance R.sub.1. The amplification
element Q.sub.1 is driven according to the comparison between the
feedback voltage V.sub.L, 2 and the set voltage V.sub.L, set. In
this way, the input value V.sub.L, set sets a constant current
value of the load current I.sub.L, which is controlled by the
linear current driver 14.
[0044] As linear current driver circuits per se are well known to
the skilled person, it is clear that the circuit 14 may be
implemented differently, as long as the basic
functionality--control of the load current I.sub.L due to linear
control of an amplification element, such as Q.sub.1--is
maintained.
[0045] The control unit 16 in the shown example is comprised of a
microcontroller 30. The microcontroller 30 may be any type of
programmable microcontroller, and, as shown in FIG. 5, preferably
include a central processing unit 50, an input/output port 52 for
receiving e. g. the set current I.sub.set and a trigger signal T as
digital signals, a non-volatile memory 54, such as a ROM, EPROM or
Flash memory for program storage, a RAM 56 for data storage and a
clock 58. As will be further explained below, the microcontroller
30 has at least three A/D converter inputs 60 for receiving
analogue voltage input signals and two D/A converter outputs 62 for
output of analogue signals (inputs and/or outputs may be
multiplexed). An example of microcontroller to be used is a NXP P
89 LPC 935.
[0046] Stored within the program memory 54 of microcontroller 30 is
a program which implements control as will be explained further on:
The controller 30 (and therefore, the running control program)
receives a set value I.sub.set as input. The program also receives
input about the current switching output voltage V.sub.1 of the
switching converter 12 and the voltage V.sub.L, 1 over the linear
current driver 14. These inputs are received as analog signals, and
are converted to digital signals using A/D converters 60 within
microcontroller 30.
[0047] As outputs, microcontroller 30 outputs a voltage signal
V.sub.1, set as a set voltage for the output voltage V.sub.1 of the
switching converter 12, and a voltage signal V.sub.L, set as a set
voltage associated with a set load current I.sub.L, set through the
linear current driver 14. Both output signals are analog signals,
output by D/A converters 62 within microcontroller 30.
[0048] The program first operates to set said load current I.sub.L
according to the received set current value I.sub.set. This is done
by applying a suited current control output V.sub.L, set. The
characteristic of linear current driver 14 is stored within
microcontroller 30, so that microcontroller 30 may directly
determine the necessary V.sub.L, set for a requested I.sub.set.
This control will now lead to the linear current driver 14
controlling the load current I.sub.L to the desired value
I.sub.set. As known for linear control, this will work very quickly
and efficiently, as long as the amplification element Q.sub.1 may
operate within its linear range, i. e. as long as the voltage
V.sub.L, 1 over the linear current driver 14 is above a lower
threshold value V.sub.threshold.
[0049] The voltage level V.sub.threshold required to keep the
amplification element Q.sub.1 within the linear range is, for the
exemplary circuit shown in FIG. 1, dependent on the load current
I.sub.L: V.sub.L, 1=I.sub.LR.sub.1+V.sub.Q1. For a bipolar
transistor, such as Q.sub.1 in the example, it is known that the
minimum voltage still allowing current I.sub.L to pass is about 0.2
V. However, at such low voltages, both amplification and speed of
Q.sub.1 are very low as compared to higher voltages, such as
V.sub.Q1 in the range of 1-2 V.
[0050] Therefore, if dynamic behavior is not important--as is the
case for a constant desired value of the load current I.sub.L--the
voltage V.sub.threshold may be chosen e.g. at V.sub.threshold=0.2
V+I.sub.LR.sub.1. Since the resistance R.sub.1 is chosen quite
small (0.3.SIGMA. in the example shown), it is possible to
calculate a constant (i.e. not dependent on the load current) value
V.sub.threshold for the maximum load current.
[0051] The program executed further, according to a basic control
function, works to control the output voltage V.sub.1 such that the
voltage V.sub.L, 1 at the linear current 14 is kept at the
threshold value V.sub.threshold.
[0052] This control strategy serves to benefit both from the
excellent dynamic behavior of linear current driver 14 and the
reduced losses of switching converter 12. By keeping the voltage
V.sub.L, 1 only at the threshold voltage V.sub.threshold for linear
operation of that device, losses here are minimized.
[0053] While this basic control strategy as explained above may
advantageously be employed for unchanging (or only slowly changing)
values of the set current I.sub.set, the control program has a
further control functionality for changing set current values
I.sub.set.
[0054] As described above, control unit 16 performs control of the
circuit 10 as closed-loop control. For a desired (constant)
L.sub.set, the closed-loop control will eventually deliver a
corresponding, necessary (constant) set value V.sub.1, set for the
switching output voltage V.sub.1. During operation, microcontroller
30 stores this information in its internal data storage 56. Data
storage 56 is organized as a look-up table of demanded load current
set values L.sub.set and determined set values V.sub.1, set for the
switching output voltage. This table is continuously updated, as
the result change (e. g. due to variations of the components of
circuit 10, or of the load etc.).
[0055] If the set value L.sub.set changes to a new value, the
program will first recall values from memory 56 to determine if a
value of V.sub.1, set is already known for the demanded I.sub.set.
If the exact value of I.sub.set is not found, the value V.sub.1,
set for the next higher I.sub.set may be used (alternatively, a
value V.sub.1, set may be determined as a linear interpolation of
the two nearest I.sub.set values). This delivers a--more or less
exact--digital value V.sub.1, set which then is used in closed-loop
control as a starting value.
[0056] As a further control functionality, the program may be used
to drive the load L according to a pulse sequence with transitions
known in advance. As shown in FIG. 1, microcontroller 30 receives
an external trigger input T which is use to indicate the
periodicity of the signal I.sub.set. At each start of a periodic
sequence (e.g. each frame in a projection application), trigger T
is shortly activated. Microcontroller 30 then stores the following
sequence I.sub.set until the next trigger T is received. For all
remaining frames, the sequence I.sub.set is pre-stored in the
memory of microcontroller 30 such that pulse timing and height
are--at least approximately--known in advance.
[0057] As shown in the top part of FIG. 3, the value I.sub.set
varies over time in pulsed manner. The signal I.sub.set thus
comprises steady portions, where the value I.sub.set remains
constant, and change portions (i. e. the rising and falling edges
of a pulse signal), where the values change. Specifically, within
FIG. 3 there are two rising edges 36, 38 and a falling edge 40 of
the signal I.sub.set.
[0058] As the switching converter 12 comprises an output
capacitance C.sub.1, the switching output voltage V.sub.1 will only
vary steadily over time, i. e. the capacitance C.sub.1 needs to be
charged respectively discharged for the voltage V.sub.1 to change.
In order to allow time for the charge process before a rising flank
36, 38 in FIG. 3, the set voltage V.sub.1, set is already raised by
the program executed in microcontroller 30 in advance of a rising
edge 36, 38.
[0059] If information is available within microcontroller 30 about
an impending rising edge 36 or 38 of I.sub.set, the set output
voltage V.sub.1, set is raised in advanced to the level required
for the next pulse. This level is retrieved from storage, as
explained above. The time period T.sub.A, by which V.sub.1, set is
raised in advanced depends on the known charging behavior of the
output capacitance C.sub.1. This behavior may be pre-programmed
within microcontroller 30 by storing the change rate of the output
voltage V.sub.1 which is achievable by converter 12. With the known
height of the rising edges 36, 38, and consequently the known
difference for the required V.sub.1, the duration of T.sub.A may be
calculated.
[0060] As shown in FIG. 3, rising of V.sub.1, set leads to a slowly
rising switching output voltage V.sub.1 (note, that in FIG. 3, the
shown voltages are simplified, giving a linearly rising voltage
V.sub.1. In a real application, the voltage may rise differently,
according to the specific, pre-programmed behavior of switching
converter 12 with its output capacitance C.sub.1).
[0061] While the output voltage V.sub.1 now rises, the linear
current driver 14 automatically controls the load current I.sub.L
to the desired, still lower, level. Thus, the voltage V.sub.L, 1
(dotted line) rises. This, of course, leads to losses within the
linear current driver 14. However, these losses are limited because
of the short time period T.sub.A.
[0062] As now the rising flank 36, 38 approaches, the switching
output voltage V.sub.1 is already at the necessary level. The
linear current driver 14 now continues to perform its function to
control the desired load current I.sub.L according to the newly set
level. The switching output voltage V.sub.1 is only subsequently
slightly adjusted in closed-loop control so that linear current
driver 14 operates within its linear range (V.sub.L,
1.gtoreq.V.sub.threshold). Thus, the dynamic behavior of linear
current driver 14 may be fully exploited for the rising flanks.
[0063] In the operating mode shown in FIG. 3, there are no special
provisions for falling flanks. At the falling flank 40, the set
voltage V.sub.1, set is reduced. During the discharge time of
capacitor C.sub.1, the actual voltage V.sub.1 is continuously
reduced. During this time, the voltage V.sub.L, 1 remains above
V.sub.threshold, so that for the limited discharge time, losses are
produced in linear current driver 14. It should be noted, that to
achieve the preferred highly dynamic behavior of the linear current
driver at the falling flank 40 (recognizable by the steeply rising
edge V.sub.L, 1 here), it is necessary to use a threshold voltage
V.sub.threshold which is slightly above the necessary minimum (e.g.
0.2 V+I.sub.LR.sub.1). Thus, for example, the threshold value could
be chosen as a fixed, slightly elevated value, e.g.
V.sub.threshold=0.5 V.
[0064] In an alternative operating mode, as shown in FIG. 4, the
same set current value I.sub.set as in FIG. 3, with rising flank
36, 38 and a falling flank 40 is to be obtained. However, a
different driving strategy is employed in order to achieve even
higher overall efficing while still obtaining a corresponding
output which closely follows the set value.
[0065] As shown in FIG. 4, before each rising flank 36, 38, but
also before each falling flank 40, the value of V.sub.1, set is
raised to a high levelThe duration of the time interval T.sub.B
before each change period (rising/falling edge 36, 38, 40) is
determined by the program according to the known
charging/discharging behavior of the switching converter 12, such
that the actual voltage V.sub.1 within the time period reaches the
desired high level.
[0066] As shown in FIG. 4, the high level to which V.sub.1, set is
raised before the change portions (rising/falling flank) is above
both the V.sub.1, set level before and after the flank. Thus, for a
falling flank 36, 38, the voltage V.sub.1 (which follows the set
value V.sub.1, set with some delay) is first raised to a high
level, and then, during the following steady portion, lowered to
the necessary minimum (again, the starting value for V.sub.1, set
in the steady portion may be retrieved from storage). Also, for the
falling flank 40, the voltage V.sub.1, set is first raised and then
lowered. The benefit of this is that the voltage V.sub.L, 1 at the
linear current driver 14 is raised to a higher level (as shown in
FIG. 4), which allows to further improve the dynamic behavior
(fast, exact control with low over-swing) of the linear current
driver 14. The losses introduced by this are only effective for a
short time (T.sub.B).
[0067] The high levels of the voltage V.sub.1, set used before each
change portion may be determined as follows: The value V.sub.1, set
needs to be high enough so that after the voltage drop at the load
L, the voltage V.sub.L, 1 is still high enough for the switching
element Q.sub.1 to have good dynamic behavior. The voltage over the
load L may, for an LED load, be calculated as a constant internal
voltage with an additional series resistance,
V.sub.LED=V.sub.0+I.sub.LR.sub.int with e.g. V.sub.0=1.5 V and
R.sub.int=0.5.SIGMA. for a red high power LED. Since the current
levels before and after the change portion are known, the desired
minimal voltage V.sub.L, 1 may thus be easily calculated.
[0068] In the following, an example will be given for a rising
flank 36, 38, where the set current value I.sub.L rises from 1 A to
2 A. The values of V.sub.1, set may then be chosen as follows:
TABLE-US-00001 steady portion steady portion before change portion
after I.sub.L = 1 A Transition I.sub.L = 2 A V.sub.LEDs = 8 V
maximum 10 V V.sub.LEDs = 10 V V.sub.L1 = I.sub.LR.sub.1 + V.sub.L1
= I.sub.L. maxR.sub.1 + V.sub.L1 = I.sub.LR.sub.1 + 0.2 V = 0.5 V 2
V = 2.6 V 0.2 V = 0.8 V V.sub.1 = 8.5 V V.sub.1 = 12.6 V V.sub.1 =
10.8 V
[0069] Thus, the program may determine the necessary levels for
V.sub.1, set during each steady portion, and also during the
transition portion preceding each change portion.
[0070] It should be noted that the program is not limited to the
exact values according to the above calculations. On one hand, it
is possible to add a certain security margin to the exact
calculated value, at least in the transition portion, to ensure
proper function (at the cost of slightly elevated losses) even in
case that the actual estimated values of V.sub.LED differ in
practice.
[0071] On the other hand, the stored model of the LED load, i. e.
values for V.sub.0 and R.sub.int, may be updated by measuring the
resulting actual value for V.sub.LEDs during control. The program
may use the measured data to obtain updated model values (in the
above example: V.sub.0, R.sub.int), and to use these updated values
for future calculations.
[0072] As explained above in relation to the operating mode shown
in FIG. 3, the dynamic behavior of the linear current driver 14
worsens if the amplification element Q.sub.1 is driven only at the
minimum level. For the operating mode shown in FIG. 3, this has
been compensated by choosing V.sub.threshold at a (slightly
elevated) constant level, e.g. V.sub.threshold=0.5 V. In the
presently explained operating mode as shown in FIG. 4, within the
steady portions, the level V.sub.threshold may even be chosen
lower. This is, because also for falling flank 40, the switching
output voltage V.sub.1 is chosen such that the voltage V.sub.L, 1
at the linear current driver is sufficient for good dynamic
behavior. Thus, the operating mode according to FIG. 4 may be
implemented to even have a higher overall efficiency.
[0073] FIG. 2 shows an alternative embodiment of the circuit 10 of
FIG. 1. Like elements refer to like parts and will not be further
described here.
[0074] The circuit 110 shown in FIG. 2 differs from the circuit 10
in that a linear current driver 114 is shown which only consists of
a transistor Q.sub.1 as the amplification element, whereas the
feedback circuit is comprised of the series resistance R.sub.1 and
a part of microcontroller 30. Microcontroller 30 has a further
input V.sub.L, 2 which serves as a feedback input directly
representing the load current I.sub.L. At its output,
microcontroller 130 delivers a base current I.sub.B to transistor
Q.sub.1 of the linear current driver 114. Thus, the program running
on microcontroller 130 also performs the closed-loop control of
I.sub.L in the digital domain, evaluating V.sub.L, 2 and providing
an appropriate current I.sub.B to control it to the desired value
I.sub.set.
[0075] FIG. 6 shows a third embodiment of a driver circuit 210.
Driver circuit 210 corresponds in large parts to driver circuit 110
according to the second embodiment (FIG. 2). Like parts are
referenced by like reference numerals and will not be further
explained in detail.
[0076] Driver circuit 210 is disposed to supply three loads
L.sub.1, L.sub.2, L.sub.3. Each load is connected in series to a
linear current driver 114, thus forming a branch. The four branches
are connected in parallel to the output voltage V.sub.1 of the
switching converter 12.
[0077] A control unit 216 controls the three branches. As in the
previous embodiments, a microcontroller 230 supplies a set output
voltage V.sub.1, set to feedback controller 26.
[0078] Also, microcontroller 230 accepts input voltages from each
of the linear current drivers 114 (comparable to input voltages
V.sub.L, 1, V.sub.L, 2 according to the second embodiment) and
delivers output currents to each of the linear current drivers 114
(corresponding to the output I.sub.B in the second embodiment).
[0079] The inputs and/or outputs provided at microcontroller 230
may be direct inputs and/or outputs. If the timing permits, the
inputs and/or outputs may also be multiplexed, so that for each
type of input and/output, only one A/D or D/A converter is actually
used.
[0080] In a first operating mode of the device according to FIG. 6,
all three loads L.sub.1-L.sub.3 are driven simultaneously. In this
case, control of the switching output voltage V.sub.1 is effected
according to the branch with the lowest voltage V.sub.L, 1 at the
linear current driver, which is controlled to be equal to
V.sub.threshold. In all other branches with less current, the
individual linear current drivers 114 limit the current according
to the desired I.sub.set.
[0081] In another operating mode, the loads are operated
substantially sequentially (i.e. with no or minimum overlap). A
corresponding example of an actual implementation is shown in FIG.
7. Here, the three loads L.sub.1-L.sub.3 are LED light sources for
a back-projection display.
[0082] The Red (R), Green (G) and Blue (B) LEDs are sequentially
driven with current pulses. The same pulse pattern is repeated with
the display frame rate (e.g. 60 Hz). The pulse frequency, however,
is substantially higher, in the shown example 50 times higher.
[0083] The differently colored LED loads require different voltages
for normal light output. As may be seen to the right of the shown
sequence, there are additionally very short current pulses in each
frame for special correction purposes, which require a higher
voltage. Since the sequence is periodically repeated, the
microcontroller of the driver circuit may select the necessary
voltages in advance.
[0084] In the lower half of FIG. 7, the lower horizontal lines show
the voltage level required for the blue (V.sub.B), green (V.sub.G)
and red (V.sub.R) LED device at normal level. The microcontroller,
however, supplies for all other current levels the corresponding
voltage level, so that also for the varying pulses the correct
current level may be achieved. In the projector shown, the current
values are chosen according to the amount of light needed for
displaying the desired image. Therefore, in each frame a different
absolute current value may be required.
[0085] It has thus been shown how the complex control task of
producing current pulses as shown in FIG. 7 may be achieved by the
relatively simple circuit shown in FIG. 6. Due to the good dynamic
properties of the linear current drivers in the three branches,
control results are excellent, even at the very short, high current
pulses in the last third of the shown sequence.
[0086] The invention has been illustrated and described in detail
in the drawings and foregoing description. Such illustration and
description are to be considered illustrative or exemplary and not
restrictive; the invention is not limited to the disclosed
embodiments.
[0087] In the claims, the word "comprising" does not exclude other
elements, and the indefinite article "a" or "an" does not exclude a
plurality. The mere fact that certain measures are recited in
mutually different dependent claims does not indicate that a
combination of these measures cannot be used to advantage. Any
reference signs in the claims should not be construed as limiting
the scope.
* * * * *